The pipes that are of interest here include any hydrocarbon duct located between a well-head (on the sea floor) and a boat intended for the storage and possibly the treatment of hydrocarbons, commonly referred to as FPSO (floating production, storage and offloading). These ducts include in particular the production lines (flowline), jumpers, spools, risers and bundles.
On the other hand, obstruction means any type of solid deposit or deposit with substantial viscosity able to be located inside the pipe, and therefore able to hinder the flow of the hydrocarbons. This can be deposits of hydrates, paraffins, minerals, asphaltenes and naphtenates. In particular, clogs formed of gas hydrate crystals can form in pipes for transporting hydrocarbons. These hydrates are formed in the presence of light hydrocarbon gases (methane, ethane, propane, etc.) and of water molecules in conditions of high pressure and low temperature. Such conditions come together in particular in the case of underwater pipes used in offshore oil and gas operations.
Of course, the appearance of obstructing elements in offshore pipes is highly undesirable. If the obstruction is partial, i.e. if it does not close the entire section of the pipe, it substantially disturbs the flow, all the more so that the formation of hydrates or of other compounds also tends to increase the viscosity of the fluid to be transported. If the obstruction is total, the pipe is completely out of service. When such circumstances arise, the operator needs effective techniques to locate where the obstruction is along the pipe and in order to destroy it. It is in particular desirable that these techniques not be intrusive, i.e. do not require opening the pipe in order to access the inside.
A known method for attacking clogs of hydrates uses a depressurisation of the pipe in order to break the thermodynamic equilibrium that favours the formation of hydrates. The depressurisation has to be obtained simultaneously on both sides of the clog, in order to balance the mechanical effect on the latter and as such prevent the clog from moving too abruptly in the pipe rather than be destroyed. This operation is not always possible, in particular in the case of multiple clogs or in the case of a partial obstruction.
The injection of chemical inhibitors can be an additional means of provoking the phenomenon of dilaceration. Commonly used agents are, for example, ethanol or methanol. They must be injected in large quantities.
Another approach is based on the action of power ultrasound transducers distributed around the pipe. GB 2440 948 A mentions removing obstructions on the inner wall of a pipe by means of ultrasound transducers distributed around the pipe and by applying multi-frequency vibrations in order to generate cavitation bubbles able to pull off elements that are obstructing the pipe.
Before attacking a clog in a pipe, it has to be able to be located. Preferably, the locating method used must be non-intrusive, i.e. not require access to the inside of the pipe, which may be impossible or highly inconvenient, especially in the deep offshore.
Known methods for detecting deposits in a pipe use a vibratory excitation generated by an actuator which provokes a response measured on a sensor located downstream of the actuator along the pipe. US 2008/0215248 A1 describes such a method for detecting bio-films that can be depositing on the walls of reservoirs or pipes that are part of industrial installations. In WO 2010/086238 A1, it is proposed to estimate the thickness of a waxy deposit inside oil pipelines by a measurement of the resonance frequency of the structure. The method described in FR 2754898 B1 seeks to evaluate the thickness of the deposit possibly formed inside the pipe using an estimation of the low-frequency vibration attenuation coefficients (less than 5 kHz) in the excited portion of the pipe.
These various methods can be delicate to implement. They do not provide a global response to the problem of locating and of destroying obstructions in offshore pipes.
There is a need for an approach that is better suited to the underwater environment.